Various surgical implants have been used in numerous forms of reconstructive and corrective surgical procedures. One example is a spinal fixation systems used to align, stabilize and/or fix a desired relationship between adjacent vertebral bodies. Such systems typically include a spinal fixation element, such as a relatively rigid fixation rod or plate, that is coupled to adjacent vertebrae by attaching the element to various anchoring devices, such as hooks, bolts, wires or screws. The spinal fixation element may take numerous forms, including a preconfigured shape specific to the implantation site within a patient. Following installation, this spinal fixation element serves to hold the vertebrae in a desired spatial relationship. The spinal fixation element may be implanted for a fixed period of time, to allow healing or fusion to occur. Additionally, the spinal fixation element may be designed for prolonged implantation during the life of a patient.
Each spinal fixation element must be anchored to various regions of a patient. For example various vertebrae sites may be designed by a surgeon as locations for a spinal fixation element. Each of these sites may vary in shape, location, composition and landscape. Therefore, a variety of anchoring devices have been developed to provide for engagement of a particular portion of the bone. Pedicle screw assemblies, for example, have a shape and size that is configured to engage pedicle bone. Such screws typically include a threaded shank that is adapted to be threaded into a vertebra and a head portion. The head portion is typically sized and orientated to allow for a spinal rod to be captured and held in position. This capture of a spinal rod is typically accomplished by providing a U-shaped recess in the head of the pedicle screw which is capable of accepting the spinal rod. Various retention mechanisms, such as a setscrew, plug, or cap, may further be employed to fix the spinal rod in place within the U-shaped recess. In use, the shank portion of each screw is then threaded into a vertebra, and once properly positioned, a fixation rod is seated through the rod-receiving portion of each screw and the rod is locked in place by tightening a cap or similar type of closure mechanism to securely interconnect each screw and the fixation rod. Other anchoring devices include hooks and other types of bone screws
In many pedicle screws, the head is designed to allow motion in along a plurality of axes relative to the shank of the threaded engagement portion. This freedom of motion of the head thereby aids in alignment and seating of a rod connecting a plurality of screws. Furthermore, the range of motion available may differ based on the direction of head displacement relative to the shank of the pedicle screw. For example, a head may have 30 degrees of motion along a first axis, and only 5 degrees of motion along a second axis. These types of polyaxial screws are generally known as “favored angle” or “biased angle” polyaxial screws.
Polyaxial screws and other implants having movable components are often difficult to manipulate during a surgical procedure, thereby requiring time consuming surgical procedures to properly orient the implant within a patient. Furthermore, surgeons oftentimes must make several attempts at proper implant insertion. To insert and remove the screw, the components of the implant must be made rigid relative to each other, to enable rotation of the shaft in a desired direction by engaging the head. Current drivers for inserting polyaxial screws accomplished this using several techniques, but are generally incapable of inserting a favored angle polyaxial screw in a manner such that the head position is maintained during insertion. Maintaining head position is essential when using a favored angle screw as the range of mobility of the head may be sufficiently small along non favored angle axis such that it may be impossible to use the implanted screw unless the head is orientated properly.